1. Field of the Invention
Embodiments of the invention relate to a touch sensing device incorporating touch sensors within a pixel array, and a display device using the same.
2. Discussion of the Related Art
A user interface (UI) is configured so that users are able to communicate with various electronic devices and thus can easily and comfortably control the electronic devices as they desire. Examples of a user interface include a keypad, a keyboard, a mouse, an on-screen display (OSD), and a remote controller having an infrared communication function or a radio frequency (RF) communication function. User interface technologies have continuously expanded to increase users' sensibilities and handling convenience. User interfaces have recently been developed to include touch UI, voice recognition UI, 3D UI, etc.
The touch UI has been used in portable information appliances such as smartphones, and has even expanded to the use of laptop computers, computer monitors, home appliances, and so on. In recent years, a technology for embedding touch sensors in a pixel array of a display panel (hereinafter, “in-cell touch sensor technology”) has been proposed. With in-cell touch sensor technology, touch sensors can be installed on a display panel without increasing the thickness of the display panel. These touch sensors may be connected to pixels through parasitic capacitance. In a driving method of the touch sensors, to reduce the mutual influence of the pixels and the touch sensors due to coupling, one frame period may be time-divided into a period for driving the pixels (hereinafter, “display driving period”) and a period for driving the touch sensors (hereinafter, “touch sensor driving period”).
In-cell touch sensor technology typically uses electrodes connected to the pixels of the display panel as electrodes for the touch sensors. In the in-cell touch technology, for example, a common electrode for supplying a common voltage to the pixels of a liquid crystal display device may be divided and used as the electrodes for the touch sensors. Although the common voltage should be the same for every pixel, the division of the common electrode into electrodes for the touch sensors makes the common voltage non-uniform on a large-sized screen, causing the deterioration of picture quality.
With reference to FIGS. 1 to 3, in the in-cell touch sensor technology, a common electrode COM may be divided into a plurality of sensor electrodes C1 to C4. The sensor electrodes C1 to C4 may each operate as touch sensors having self-capacitance. Sensor lines L1 to L4 may be connected to the sensor electrodes C1 to C4, respectively. The capacitance of the touch sensors may increase when a conductive object, such as a finger, touches the touch screen. Consequently, touch input can be detected by measuring changes in capacitance caused by a touch.
During a display driving period Td, the common voltage Vcom for the pixels may be supplied to the sensor electrodes C1 to C4 through the sensor lines L1 to L4. During a touch sensor driving period Tt, a sensor driving signal Tdrv may be supplied to the sensor electrodes C1 to C4 through the sensor lines L1 to L4.
The length of the sensor lines L1 to L4 may vary depending on where the touch sensors may be located. Accordingly, the delay time of the common voltage Vcom applied to the sensor electrodes C1 to C4 may vary depending on the locations of the touch sensors, due to variations in length between the sensor lines L1 to L4, thus making picture quality nonuniform.
For example, as shown in FIG. 3, the delay time of the common voltage Vcom applied to the first sensor electrode C1 through the first sensor line L1 may be longer than the delay time of the common voltage Vcom applied to the fourth sensor electrode C4 through the fourth sensor line L4. That is, the first sensor line L1 involves a longer RC delay than the fourth sensor line L4 because the first sensor line L1 may be larger in length than the fourth sensor line L4. Hence, even when the same voltage may be applied to the first and fourth sensor lines L1 and L4, the voltage of the first sensor electrode C1 may be lower than the voltage of the fourth sensor electrode C4. Due to RC delay, the delay time of the sensor driving signal Tdrv also may vary depending on the locations of the touch sensors.
For a large-screen display device, the differences in length between the sensor lines L1 to L4 become larger. Accordingly, in-cell touch sensor technology might suffer from non-uniformity in the common voltage Vcom applied through the sensor electrodes C1 to C4 during the display driving period Td. This can result in deterioration of the picture quality of the display device.
Large-screen display devices may have large parasitic capacitance due to coupling between in-cell touch sensors and pixels. This may increase the size and resolution of a touch screen using the in-cell touch sensors and also increase the parasitic capacitance connected to the in-cell touch sensors, leading to a reduction in touch sensitivity and touch recognition accuracy. Therefore, there is a desire to reduce the parasitic capacitance of touch sensors in order to apply the in-cell touch sensor technology to the touch screen of a large-screen display device.
The common voltage Vcom of the pixels may vary depending on the screen position, because of RC delay variations on the display panel. Also, the common voltage Vcom may vary when the display panel is driven in a time-division manner in a display driving period and a touch sensor driving period, separately, or when the display driving period starts immediately after the touch sensor driving period. Because such common voltage variations may cause luminance variations between the pixels, horizontal lines may appear on the screen.
One way to compensate for the variations in common voltage Vcom is to compensate common voltages by sensing common voltage changes fed back from the common electrode of the display panel. An example method of common voltage feedback compensation was proposed by the present applicant in Korean Laid-Open Patent Nos. 10-2006-0077951, filed on Jul. 5, 2006, and 10-2013-0139679, filed on Dec. 23, 2013. Such a method may require a feedback line for connecting the common electrode and a feedback compensation circuit. However, the feedback line cannot be connected to each of the sensor electrodes, which may be divided from the common electrode as shown in FIG. 1, so the method of common voltage compensation may not be applicable.